This application relates generally to electrooptical devices and liquid crystal materials employed in such devices. More particularly, the invention relates to bistable and analog electrooptical devices employing ferroelectric liquid crystal materials.
Liquid crystals have found use in a variety of electrooptical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively “floppy” tails. Such LC molecules are generally rod-like in shape with the rigid core generally along the long axis of the molecule. Ferroelectric liquid crystal (FLC) materials have been prepared by the introduction of one or more chiral nonracemic LC molecules having one or more stereocenters in at least one of the tails to introduce chirality. The first FLC compound to be characterized was DOBAMBC which contains an (S)-2-methylbutyloxy chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarzation of −3 nC/cm2.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of N. A. Clark and S. T. Lagerwall (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures using FLC materials, the so-called Surface-Stabilized FLC (SSFLC) devices, having not only high speed, but which also exhibit bistable, threshold sensitive switching. Such properties make FLC-based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
It is, however, well known in the art of FLC materials and devices that a typical FLC device does not exhibit true optical bistability, that is, the memory or the zero applied field orientation of the optic axis of the SSFLC device is typically different from that of its driven orientation. Descriptions of the construction and operation of a conventional bistable FLC device can be found, for example, in U.S. Pat. Nos. 5,748,164 and 5,808,800. The FLC materials used in these conventional devices exhibit smectic layer spacing shrinkage at the smectic A to smectic C transition and further into the smectic C phase. The most significant consequence of the decrease in smectic layer thickness, is the formation of chevron smectic layer structures. In addition to inducing many defects, formation of such chevron structures, in effect, adds an extra interface at the chevron interface which is a nominally planar interface roughly parallel to the plane of the FLC film. This extra interface is internal to FLC materials, and together with the two surfaces bounding the FLC materials and the external electric field, determines the orientation of the optic axis of the FLC device. The added constraint imposed by the chevron interface is that the orientation of the optic axis of the FLC devices under an applied electric field depends on the strength of the applied field, and is, thus, different from the memory orientation of the device in the absence of the applied field.
FIG. 1A illustrates a smectic C* chevron interface. See, for example, Rieker, T. et al. (1987) Physical Rev. Letts. 59(23):2658 for a discussion of chevron layer structure in SSFLC cells. FIG. 2A schematically illustrates a typical electrooptical response (output light intensity as a function of applied voltage) of a conventional bistable FLC device. This conventional bistable device does not exhibit a true bistable switching and does not exhibit analog behavior. FLC compositions exhibiting bookshelf geometry (FIG. 1B) will, in contrast, be substantially chevron-free when aligned in SSFLC devices and exhibit true bistable electrooptical response as schematically illustrated in FIG. 2B.
Much attention has focused on the construction of FLC electrooptical devices with true optically bistability which are extremely desirable in practical applications to achieve stable memory performance, high contrast ratio, wide viewing angle and high speed response. However, only a few FLC materials have been identified which exhibit true bistability. A small class of naphthalene-based LCs were reported to be useful for preparation of FLC mixtures exhibiting optical bistability (Mochizuki et al. (1991) Ferroelectrics 122:37-51, U.S. Pat. No. 5,169,556, EP published application 405,868 (published Feb. 1, 1991) and U.S. Pat. No. 5,348,685.) These FLC materials are said have bookshelf geometry and to exhibit no smectic layer spacing shrinkage at the smectic A (SmA) to the chiral smectic C (SmC*) transition and into the SmC* phase range, unlike many conventional FLC materials. U.S. Pat. Nos. 5,568,299, 5,856,815 and 5,943,112 report applications of the naphthalene-based FLCs of U.S. Pat. Nos. 5,169,556 and 5,348,685.
Additional naphthalene-core LCs are reported to provide improvement in response times and/or temperature dependency of response time in U.S. Pat. No. 5,861,108. While this patent discloses numerous naphthalene-core LC molecules and LC molecules with related structures, it does not indicate that any of the disclosed LC molecules provide chevron-free bistable FLCs.
U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 report achiral LC compounds having perfluoroether terminal groups exhibiting smectic phases or latent smectic phases that are said to provide “reduced temperature dependence of the smectic interlayer spacing” and “spontaneous generation of a bookshelf layer structure ideal for a ferroelectric liquid crystal device.” Preferred chiral LCs of these patents have a phenylpyrimidine core. A number of LC molecules have been reported to be useful in combination with these achiral bookshelf LCs.
U.S. Pat. Nos. 5,474,705, 5,702,637 and 5,972,241, as well as published EP application EP 736,078 (published Jun. 24, 1998) report chiral LC compounds also having a perfluoroether terminal portion or a chiral fluorinated terminal portion with preferred LC compounds having phenylpyrimidine cores. These patents report that the chiral LC molecules disclosed can be admixed with the achiral fluoroether-containing compounds of U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 to exhibit “reduced temperature dependence of the smectic interlayer spacing” and “spontaneous generation of a bookshelf layer structure ideal for a ferroelectric liquid crystal device.
U.S. Pat. Nos. 5,658,491, 5,855,812 and 5,928,562 report a process for controlling cone tilt angle in tilted smectic FLC compositions. The compounds disclosed contain fluoroether or fluoroalkyl groups in the LC tail. The patents further report that the compounds useful in the invention can be admixed with the achiral fluoroether-containing compounds of U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 to exhibit “reduced temperature dependence of the smectic interlayer spacing” and “spontaneous generation of a bookshelf layer structure.”
U.S. Pat. Nos. 4,886,619, 5,082,587, 5,399,291, 5,399,701 report chiral and achiral LC molecules having tilted smectic mesophases or latent tilted smectic mesophases and having fluorocarbon terminal portions. The LC compounds disclosed have structural features in common with bookshelf LCs of U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650, however, none of the LC compounds disclosed are specifically identified as useful for preparation of chevron-free bistable FLCs.
U.S. Pat. Nos. 5,750,214 and 5,858,273 report liquid crystal devices with certain alignment control, which is said to be useful in improving a switching characteristic of a chiral smectic liquid crystal composition having bookshelf structure. The patents refer to the use of FLC compositions in the method in which at least one component of the FLC composition has a fluorocarbon terminal portion. The patents refer specifically to the use of compounds of bookshelf LCs of U.S. Pat. No. 5,262,082.
U.S. Pat. Nos.6,019,911 and 6,007,737 report various liquid crystal compositions having structures related to the naphthalene and phenyl pyrimidines that are noted above to exhibit spontaneous generation of bookshelf structure. However, none of the LC compounds disclosed in these patents is identified as exhibiting bookshelf structure or as useful in the preparation of chevron-free FLCs.
In the field of analog FLC devices, a so-called ‘V-shaped’ switching has been reported in a class of FLCs known to produce antiferroelectric phases. Antiferroelectric LCs (Chandani et al. (1988) Jpn. J. App. Phys. 27(5):L729-L732) exhibit three stable states and are characterized by a distinct threshold and double hysteresis that generates a memory effect in the driven states. A typical electrooptic response of an antiferroelectric LC is schematically illustrated in FIG. 3A and a V-shaped switching response is schematically illustrated in FIG. 3B.
V-shaped switching is a thresholdless (or low threshold), hysteresis-free (or low hysteresis) switching effect that was first reported by Fukuda A. (1995) Asia Display'95, Proceedings of the 15th International Display Research Conference 61:177 and by Inui et al. (1996) J. Mater. Chem. 6:671 in a three component antiferroelectric LC mixture of compounds A:B:C (40:40:20 mass %) see Scheme 1, where * indicates an asymmetric carbon. It was later reported by Seong et al. (1997) J. Appl. Phys. 36:3586-3590 that compound A in this mixture when homogeneously aligned in an LC cell exhibited V-shaped switching in an antiferroelectric phase at certain temperatures. The only known test of V-shaped switching is the actual observation of the high susceptibility analog effect in FLC cells.
A “Thresholdless” antiferroelectric effect has been reported by some researchers (Fukuda, A. (1995) Asia Display '95 Proceedings of the 15th Int'l Display Research Conference 61:177 and Inui, S. et al. (1996) J. Mater. Chem 6:671) and a “ferrielectric” effect by yet other researchers (E. Gorecka et al. (1990) Jap. J. Appl. Physics 29(1):L131-L-137; Booth et al. (1996) Liquid Crystals 20(6):815-823). These effects have also been associated with antiferroelectric LC molecules. It is believed that both of these effects are substantially the same a V-shaped switching.
U.S. Pat. No. 5,942,155 reports a siloxane LC molecule that exhibits an antiferroelectric LC phase having little or no hysteresis and low threshold voltage.
U.S. Pat. Nos. 6,057,007, 6,084,649, WO 99/33814 (published Jul. 8, 1999) and WO 00/31210 (Published Jun. 2, 2000) report tristable liquid crystal devices comprising a titled smectic or induced tilted smectic LC composition. Many of the LC molecules specifically exemplified have phenylpyrimidine cores and a chiral (U.S. Pat. No. 6,057,007) or achiral (U.S. Pat. No. 6,084,649) terminal fluorocarbon group. Compositions disclosed are reported to exhibit low threshold, low hysteresis switching “approaching the ideal ‘V-shaped’ switching. However, data presented in the listed U.S. patents (specifically in Table 2) report only two LCs (both in U.S. Pat. No. 6,057,007) with zero hysteresis. The structures of the phenylpyrimidine LC molecules reported to exhibit no hysteresis on switching are illustrated in Scheme 2.
The ferrielectric effect was first reported in 4-(1-methylheptyoxycarbonyl)phenyl-4-(4′-octyloxybiphenyl)carboxylate (MHPOBC) by Gorecka, E. et al. (1990) supra. Booth wt al. (1996) Liquid crystals 20(6):815-823 also report ferrielectric LCs.
U.S. Pat. No. 5,728,864 reports certain chiral LCs having ferrielectric phases comprising a chiral ester tail group of structure:—COOCH—CH*(A)—(CH2)mOCnH2+1, where * indicates an asymmetric carbon, A can be —CF3, or —C2F5, and m and n are integers ranging from 2-4. The structure of the disclosed ferrielectric LC is given in Scheme 3.
U.S. Pat. No. 6,002,042 reports chiral “swallow-tailed” LC compounds illustrated in Scheme 4 with a trifluoromethyl substituted chiral tail group which have an antiferroelectric phases or ferrielectric phase showing V-shaped optical response.
U.S. Pat. Nos. 6,001,278, 5,938,973 and 5,976,409 report achiral swallow-tailed LC compounds that can be combined with chiral ferrielectric LC compounds to obtain ferrielectric LCs exhibiting V-shaped optical response. The swallow-tailed LC compounds disclosed have one branched alkyl ester tail with various benzoate, and phenyl benzoate cores with 1-3 phenyl rings, which may be substituted at certain ring positions with halides (particularly fluorines) see Scheme 5. Swallow-tailed LCs are also reported by Heinemann, S. et al. (1993) Mol. Cryst. Liq. Cryst. 237:277-283, Heinemann, S. et al. (1993) Liquid Crystals 13(3):373-380 and Booth, C. J. et al. (1996) Liquid Crystals 20(4): 387-392.
U.S. Pat. Nos. 5,340,498, 5,985,172, 5,980,780, 6,001,278, and 6,018,070 variously report chiral and achiral LC molecules that are said to be useful in the formation of antiferroelectric LCs. LC compounds of these patents have an ester tail of formula:—COO—CH(A)-ether (or alkyl), where A can variously be H, —CF3, —CH3, or —C2H5 (dependent upon the structure of the rest of the molecule) and the carbon of the —CH(A)— moiety can be chiral. The tail group is similar in structure to chiral tail groups of ferrielectric LCs. However, these patents do not specifically report the presence of any of V-shaped switching, ferrielectric effect or thresholdless antiferroelectric effect in any of the disclosed compounds.
Antiferroelectric LCs are also reported in the following references: JP-A-1-213390, JP-A-1-316339, JP-A-1-316367, JP-A-1316372, JP-A-2-28128, (1989) Liquid Crystals 6:167, Chandani et al. (1988) Jap. J. Applied Physics 27:L729-732, Chandani et al. (1989) Jap. J. Applied Physics 28:L1261-1264, Chandani et al. (1989) Jap. J. Applied Physics 28:L1265-1268, Chandani et al. (1989) Jap. J. Applied Physics 28:L119-120, Johno et al. (1990) Jap. J. Applied Physics 29: L111-114, Nakagawa A. (1991) Jap. J. Applied Physics 30:L1759-1564.
Liquid crystal compounds and compositions that exhibit bookshelf structure useful in constructing electrooptical devices with true bistable optical response or that exhibit V-shaped switching (low or no threshold and low or no hysteresis) useful in constructing analog electrooptical devices are of significant interest in the field. Liquid crystals currently known in the art to exhibit these desirable electrooptical properties represent a relatively narrow range of chemical structures. It is of great interest in the art to expand the range of structures of liquid crystals compounds that exhibit true bistable optical response or V-shaped switching to facilitate additional improvements in other desirable properties of materials and adaptation of materials for use in different applications. Properties that it would be desirable to improve or control include, among others, chemical (including photochemical) stability, viscosity, compatibility with other liquid crystal compounds to form mixtures, wide and useful operating temperature range, tilt angle, switching speed, spontaneous polarization, and birefringence.